1. Field of the Invention
The present invention relates to a semiconductor laser device including a semiconductor laser chip in which coating films are formed for an emission surface and a reflection surface, which are opposing end surfaces of a resonator.
2. Description of Related Art
A semiconductor laser element used in an optical disk drive generally includes a built-in photodiode for detecting laser power, in addition to a semiconductor laser chip, and laser power is controlled based on a current value (monitor current) converted by the photodiode through photoelectric effect. Inexpensive photodiodes which use silicon (Si) have conventionally been employed in 780 nm-range semiconductor laser elements for CDs and 650 nm-range semiconductor laser elements for DVDs.
In 400 nm-range semiconductor laser devices, which are for the next generation Blu-ray disks, however, the temperature dependence of semiconductor laser devices causes the wavelength of an output laser to grow longer as the ambient temperature rises, despite laser power being kept constant. The resultant problem is wild fluctuations in monitor current detected by the photodiode.
As illustrated in FIG. 12, one cause of this is that the sensitivity to wavelengths in converting light into a current varies greatly depending on Si used as the material of the photodiode.
It is consequently a common practice in conventional semiconductor laser devices with a built-in photodiode to take the fact that the photodiode sensitivity varies depending on the wavelength as a given and to correct the sensitivity considering the wavelength dependence of the monitor current on the control side.
However, the semiconductor laser chip itself experiences fluctuations in band gap due to changes in ambient temperature, causing the wavelength of its emitted laser to vary. Correcting the photodiode sensitivity alone is thus not enough to achieve specifications capable of auto power control (APC) driving (a ±5% power change in response to a change in ambient temperature).